Lincoln/Mercury Repair: 1993 Mercury Villager, mercury villager, circulation problem

Question
I have a 1993 Mercury Villager and my problem is with my heater.  When the car is running and moving, the heater works and blows warm air.  But when I stop at a stop sign or traffic light, it blows cold air.  I took it in to a shop, and nothing came up on the computer.  They flushed the radiator but the problem remains.  Just wondered if you ever heard of this before as no one else seems to have.  

Thank you in advance.  

Answer
you need to find out if it is a coolant circulation problem or a heater damper door problem. tell the mechanic to check if the damper doors (the flaps that direct the air where to go) are closing at idle. if so than there is a problem with the vacume side of the switch or a vacume line from the engine. the most common fault is lime build up in the heater core. no amount of flushing has ever unplugged it. if this is the case than you need a heater core. you should do a thermostat at the same time. with a restriction in the system it works the thermostat very hard and will usually make the spring weak.